Equilibrium Potentials of Membrane Electrodes ( glass electrode / pH ) JUI

A simple thermodynamic theory of the equilibrium potentials of membrane electrodes is formulated and applied to the glass electrode for measurement of pH. The new formulation assumes the selective adsorption or binding of specific ions on the surface of the membrane which may or may not be permeable to the ion, and includes the conventional derivation based on reversible ion transport across membranes as a special case. To test the theory, a platinum wire was coated with a mixture of stearic acid and methyl-tri-n-octyl-ammonium stearate. When this coated electrode was immersed in aqueous phosphate solution, its potential was found to be a linear function of pH from pH 2 to 12 with a slope equal to the theoretical value of 59.0 mV per pH unit at 240. Since its modest debut in the form of a glass electrode early this century (1, 2), the ion-selective membrane electrode has gradually risen to become one of the most important analytical tools in chemical as well as biomedical science and technology. Contemporary theories of ion-selective membrane electrodes often involve the quantitative treatment of charge and matter transport based on an assumed molecular mechanism. Although these kinetic treatments (3-5) and equivalent circuit models (6) have been very valuable in helping us to understand the characteristics of electrode response, they lack the generality of a thermodynamic theory of electrode potentials. As the variety of useful ion-selective membrane electrodes continues to multiply, so does the number of relevant molecular mechanisms. Consequently, it becomes increasingly difficult to develop a general kinetic treatment applicable to all ion-selective membrane electrodes. On the other hand, a purely thermodynamic theory is independent of the detailed molecular mechanism and, hence, should be more widely applicable. The customary thermodynamic treatment of membrane potential is based upon the following condition of membrane equilibrium first deduced by Gibbs (7): $il + ZiFV, = JUi2 + ZiFV2 membranes with tritium-labeled solutions gave negative results (9). When the inner solution of a glass electrode is removed and replaced by mercury (10) or a vacuum-coated platinum film, the resulting assembly still functions as a glass electrode with no noticeable change in sensitivity and selectivity. These observations cast doubt on the assumed applicability of Eq. 1 to glass electrodes, and a reformulation of the theory of electrode potentials seems to be in order.